The present invention relates to an absorbent structure in an absorbent article, such as a diaper, an incontinence guard, a sanitary napkin or like article. The structure includes a liquid-acquisition and liquid-dispersing core which is comprised of high bulk, porous material and which extends over at least a substantial part of the structure in its longitudinal direction and is in liquid communication with a liquid storage part that surrounds the liquid-acquisition and liquid-dispersing core at least along the long sides of said core and has an effective mean pore size that is smaller than the effective mean pore size of the liquid-acquisition and liquid-dispersing core.
An absorbent structure in an absorbent article, such as a diaper, an incontinence guard or a sanitary napkin intended for one-time use only is normally comprised of one or more layers of hydrophilic fibres, normally cellulose fluff pulp. The absorbent structure will often include superabsorbents, which are polymers that are capable of absorbing many times their own weight in water or body fluid. The absorbent structure may also include other components for improving its liquid-dispersion properties or enhancing its coherency and ability to resist deformation in use.
One problem encountered with absorbent structures of this kind, and then particularly in diapers and incontinence guards that are intended to acquire and absorb relatively large volumes of liquid, is that they often leak before their total absorption capacity has been utilized to the full. Because large volumes of liquid are discharged within the space of a few seconds when urinating, the absorbent body will often be temporarily saturated with liquid locally in the so-called liquid-acquisition zone, because the liquid does not have time to disperse to other parts of the absorbent body, such that any further liquid discharged will leak from the diaper. Naturally, such prior leakage before the total absorption capacity has been used to the full is highly irritating to both the user and his/her minder.
Various methods of increasing the instantaneous liquid-acquisition ability of such articles in the liquid-acquisition zone are known to the art, e.g. by providing cavities or regions of lower density in said zone; see, for instance, U.S. Pat. No. 4,413,996 and EP-A-0 254 476 in this regard. It is also known to provide a layer of porous material immediately beneath the liquid-permeable casing material of the article, for instance a layer of synthetic fibre wadding, a layer of nonwoven material, or a layer of foam that can take-up large volumes of liquid over short periods of time and temporarily store this liquid until it has had time to be absorbed in the remainder of the absorbent body.
GB-A-2 124 907 describes an absorbent structure in which a layer of high bulk, porous material is bedded in the absorbent body. This layer extend over a substantial part of the absorbent body in its longitudinal and transverse directions. Liquid is taken-up in this layer of porous material and dispersed therealong, and is gradually absorbed by the surrounding absorbent material.
U.S. Pat. No. 4,449,979 describes a loosely compacted cellulosic fibers absorbent core structure of substantially rectangular shape. The structure should readily accept and wick liquid while retaining a high liquid holding capacity. This is achieved by increasing the density of the structure from the centre to the transverse ends. It is essential that the density in a cross direction is substantially constant on any given line. The density of the structure varies according to the given examples between 0.040 and 0.062 gm/cm3 for the uncompressed centre part and between 0.063 and 0.100 gm/cm3 for the most compressed transverse ends. Previous tries to densify a portion of an absorbent bat to increase the wicking have resulted in a reduction of the liquid holding capacity and in rigid densified portions having a tendency to break when the batt is flexed. According to U.S. Pat. No. 4,449,979 this problem is solved by providing an increasing density gradient only from the central portion towards the transverse ends.
However, there is still space for improvement with regard to the liquid-acquisition and liquid-dispersing ability of the structure, particularly with respect to products that are intended for acquiring large volumes of liquid within a short space of time, as is often the case with incontinence products intended for adult incontinence, for instance.
The object of the present invention is to provide an absorbent structure of the kind defined in the introduction that is able to acquire large volumes of liquid rapidly, even in the case of repeated wetting, and spread the liquid towards unused parts of the absorbent body. This object is achieved in accordance with the invention with an absorbent structure of the kind defined in the introduction in which the liquid-absorbent material that surrounds the liquid-acquisition and liquid-dispersing layer has a density that increases outwards towards the longitudinally extending side edges of the absorbent structure.
The fact that it is possible to obtain an improvement of the acquisition and storing properties by increasing the density of the storage part outwards towards the longitudinally extending side edges is surprising in view of U.S. Pat. No. 4,449,979. The explanation may be that the core material discussed in U.S. Pat. No. 4,449,979 is fragile and cannot swell owing to too strong binding forces between the fibers and/or too weak forces to break the network. As the thinner pores of course have a lower storing capacity than larger pores, the dense parts of the core material of U.S. Pat. No. 4,449,979 will have a lower holding capacity. However, when using a material which can swell, such as dry formed flash-dried cellulose fibers or a material containing a superabsorbent, the network will have the ability to swell when absorbing liquid, thus increasing both pore diameters and the storing capacity. Non utilized areas of the storage layer which have not yet absorbed any liquid lie near the utilized liquid containing area. These non utilized areas contain fine pores which try to wick and drain the liquid xe2x80x9cstoredxe2x80x9d in the utilized area. Thus, both a better liquid transportation and a high storing capacity are obtained according to the present invention.